Separation type unit pixel of 3-dimensional image sensor and manufacturing method thereof

ABSTRACT

A separation type unit pixel of an image sensor, which can control light that incidents onto a photodiode at various angles, and be suitable for a zoom function in a compact camera module by securing an incident angle margin, and a manufacturing method thereof are provided. The unit pixel of an image sensor includes: a first wafer including a photodiode containing impurities having an impurity type opposite to that of a semiconductor material and a pad for transmitting photoelectric charge of the photodiode to outside; a second wafer including a pixel array region in which transistors except the photodiode are arranged regularly, a peripheral circuit region having an image sensor structure except the pixel array, and a pad for connecting pixels with one another; and a connecting means connecting the pad of the first wafer and the pad of the second wafer. Accordingly, manufacturing processes can be simplified by constructing the upper wafer using only a photodiode and the lower wafer using the pixel array region except the photodiode, and costs are reduced since transistors are not included in the upper wafer portion, which in turn cannot affect the interaction with light.

TECHNICAL FIELD

The present invention relates to a unit pixel of an image sensor, andmore particularly, to a unit pixel of image sensor in which a photodiodeis separated from a pixel array region and a manufacturing methodthereof.

BACKGROUND ART

Pixels used in conventional image sensors are generally classified as3-transistor pixels, 4-transistor pixels, or 5-transistor pixelsaccording to the number of transistors included therein.

FIGS. 1 to 3 show a typical pixel structure used for an image sensor,according to the number of transistors;

FIG. 1 shows a 3-transistor structure. FIGS. 2 and 3 show a 4-transistorstructure.

As shown in FIGS. 1 to 3, a fill factor that is the area occupied by thephotodiode over the entire area of the pixel is naturally reduced due tothe existence of transistors in a pixel circuit. In general, the fillfactor of a diode ranges from 20 to 45%, considering capability of eachsemiconductor manufacturing process. Accordingly, light that is incidentonto the rest area corresponding to about 55-80% of the entire area ofthe pixel is lost.

To minimize the loss of optical data, a microlens is used for each unitpixel in a manufacturing process of the image sensor so that the opticaldata can be condensed onto the photodiode of each pixel. A microlensgain is defined as an increment of the sensitivity of a sensor using themicrolens with respect to the sensitivity of the image sensor withoutthe microlens.

Given that the fill factor of a common diode is about 30's %, themicrolens gain is 2.5-2.8 times of the sensitivity of the image sensorwithout the microlens. However, a pixel size has decreased to 4 μm×4 μm,and even to 3 μm×3 μm. Further, with an emergence of a small-sized pixelof 2.8 μm×2.8 μm or 2.5 μm×2.5 μm, starting from when the pixel size is3.4 μm×3.40 μm, the microlens gain significantly drops from 2.8 times to1.2 times of the sensitivity of the image sensor without the microlens.This is caused by diffraction phenomenon of the microlens. The severityof diffraction phenomenon is determined by a function of a pixel sizeand a position of the microlens.

As the pixel size gradually decreases, the severity of diffractionphenomenon of the microlens increases, thereby dropping the microlensgain equal to or less than 1.2 times of the sensitivity of the imagesensor, which results in a phenomenon where the light condensation seemsto be unavailable. This is newly being recognized as a cause ofsensitivity deterioration.

In general, the decrease of the pixel size of the image sensor resultsin the decrease of the area of the photodiode. The area of thephotodiode is closely related to the amount of available electric chargeof the photodiode. Accordingly, the amount of available electric chargedecreases when the size of the photodiode decreases. The amount ofavailable charge of the photodiode is a basic factor for determining adynamic range of the image sensor, and therefore the decrease of theamount of available electric charge directly affects the image qualityof the sensor. When the image sensor of which the pixel size is lessthan 3.2 μm×3.2 μm is manufactured, its sensitivity decreases, and thedynamic range of the sensor with respect to the light also decreases,thereby deteriorating the image quality.

An external lens is used in the process of manufacturing a camera moduleusing the image sensor. In this case, light is substantiallyperpendicularly incident onto a center portion of a pixel array.However, the light is less perpendicularly incident onto edge portionsof the pixel array. When an angle starts to deviate from the verticalangle by a predetermined degree, the light is condensed onto themicrolens which is out of the area pre-assigned for condensation of thephotodiode. This generates a dark image, and more seriously, when thelight is condensed onto a photodiode of an adjacent pixel, chromaticitymay change.

Recently, with the development of the image sensor having from 0.3million pixels and 1.3 million pixel to 2 million pixels and 3 millionpixels, a dynamic zoom-in/zoom-out function as well as an automaticfocus function are expected to be included in a compact camera module.

The features of the functions lie in that the incident angle of thelight significantly changes at edge portions while each function isperformed. The chromaticity or brightness of the sensor has to beindependent of changes in the incident angle. With the decrease of thepixel size, the sensor cannot cope with the changes in the incidentangle. At present, the sensor can support the automatic focus function,but the sensor can not support the dynamic zoom-in/zoom-out function.Therefore, it is difficult to develop a compact camera module providinga zoom function.

DETAILED DESCRIPTION OF THE INVENTION Technical Goal of the Invention

In order to solve the aforementioned problems, an object of the presentinvention is to provide a separation type unit pixel of an image sensorof which sensitivity drops far lesser than a conventional case in themanufacturing of a minute pixel, capable of controlling light incidentonto a photodiode at various angles, and providing a zoom function in acompact camera module by securing an incident angle margin, and amanufacturing method thereof.

Disclosure of the Invention

According to an aspect of the present invention, there is provided aseparation type unit pixel having a 3D structure for an image sensorincluding: a first wafer including a photodiode containing impuritieshaving an impurity type opposite to that of a semiconductor material anda pad for transmitting photoelectric charge of the photodiode tooutside; a second wafer including a pixel array region in whichtransistors except the photodiode are arranged regularly, a peripheralcircuit region having an image sensor structure except the pixel array,and a pad for connecting pixels with one another; and a connecting meansconnecting the pad of the first wafer and the pad of the second wafer.

According to another aspect of the present invention, there is provideda manufacturing method of a separation type unit pixel having a 3Dstructure for an image sensor, the method including: (a) constructing afirst wafer including only a photodiode formed by implanting impurityions into a semiconductor substrate; (b) constructing a second waferincluding a pixel array region excluding the photodiode and a peripheralcircuit region; (c) arranging the first wafer and the second wafer upand down for pixel array arrangement; (d) adhering a pad of a unit pixelon the first and second wafer arranged up and down; and (e) forming acolor filter on the first wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 show a structure of a pixel according to the number oftransistors typically used for an image sensor;

FIGS. 4 to 7 are circuit diagrams showing a structure of a separationtype unit pixel for an image sensor, including a photodiode and fourtransistors, according to the present invention;

FIGS. 8 to 11 are circuit diagrams showing a structure of a separationtype unit pixel for an image sensor, including a photodiode and fourtransistors, according to an embodiment of the present invention;

FIG. 12 shows a physical structure of a separation type unit pixel foran image sensor according to an embodiment of the present invention;

FIG. 13 shows a physical structure of a separation type unit pixel foran image sensor according to another embodiment of the presentinvention;

FIG. 14 shows a separation type unit pixel for a 3-dimensional imagesensor according to an embodiment of the present invention;

FIG. 15 shows a separation type unit pixel having a 3D structure for animage sensor according to another embodiment of the present invention;

FIG. 16 is a flowchart of a method of manufacturing a separation typeunit pixel having a 3D structure for an image sensor according to thepresent invention; and

FIG. 17 shows a method of arranging a first wafer and a second waferwhen manufacturing a separation type unit pixel having a 3D structurefor an image sensor.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present will be described in detail with reference toaccompanying drawings.

FIGS. 4 to 7 are circuit diagrams showing a structure of a separationtype unit pixel for an image sensor, including a photodiode and fourtransistors, according to the present invention, in which a photodioderegion is separated from a pixel array region including fourtransistors.

It is shown that a pixel array region having a four transistor (4T)structure including a transmission transistor, a reset transistor, asource follower transistor, and a selection transistor is separated fromthe photodiode.

Here, methods of arranging the pixels having the 4T structure arevarious.

A structure of the separation type unit pixel of the image sensoraccording to the present invention may be employed for N-type and P-typeMOS transistors.

FIGS. 8 and 9 are circuit diagrams showing a structure of a separationtype unit pixel for an image sensor according to an embodiment of thepresent invention in which a photodiode region is separated from a pixelarray region including three transistors.

It is shown that the pixel array region having a three transistor (3T)structure including a reset transistor, a source follower transistor anda selection transistor is separated from the photodiode.

FIGS. 10 and 11 are circuit diagrams showing a structure of a separationtype unit pixel for an image sensor according to another embodiment ofthe present invention.

It is shown that a region where the photodiode and the reset transistorare formed is separated from a pixel array region where the rest of thetransistors except the reset transistor in the 3T structure are formed.

Here, methods of arranging the pixels having the 3T structure arevarious. A structure of the separation type unit pixel of the imagesensor according to the present invention may be employed for N-type andP-type MOS transistors.

FIG. 12 shows a physical structure of a separation type unit pixel foran image sensor according to an embodiment of the present inventionwhich includes first and second wafer 10 and 20.

The first wafer 10 includes the photodiode 14 having a P-typesemiconductor structure by implanting impurities into a semiconductorsubstrate and a pad 17 transmitting photoelectric charge of thephotodiode 14 to outside.

The second wafer 20 includes a pixel array region where the circuitelements except the photodiode including transistors are regularlyarranged, a peripheral circuit region, and a pad 21 connecting pixelswith one another.

In the pixel array region, the rest of the circuit elements constitutinga pixel (i.e., a transmission transistor 22, a reset transistor 23, asource follower transistor 24 and/or a selection transistor 25) areregularly arranged. In the peripheral circuit region, a circuit forreading a signal of the image sensor, a correlated double sampling (CDS)circuit, a circuit for processing a general analogue signal, otherdigital control circuit, and a digital circuit for processing an imagesignal are included.

FIG. 13 shows a physical structure of a separation type unit pixel foran image sensor according to another embodiment of the present inventionin which the second wafer 20 has a 3T structure.

Specifically, in the pixel array region of the second wafer 20, the restof the circuit elements constituting a pixel (i.e., a reset transistor23, a source follower transistor 24 and/or a selection transistor 25)are regularly arranged.

FIG. 14 shows a separation type unit pixel having a 3D structure for animage sensor according to an embodiment of the present invention inwhich the first wafer 10 is separated from the second wafer 20.

The first wafer 10 includes a color filter 12 and the photodiode 14.

The second wafer 20 includes a pixel array region where elementsincluding transistors are regularly arranged and a peripheral circuitregion constituting an image sensor structure including the rest of thepixel elements except the pixel array.

The first and second wafer 10 and 20 are connected to conductor layers17 and 21 for external connection.

The first wafer portion 10 will now be described in detail.

The first wafer 10 includes the color filter 12 allowing each pixel todisplay a specific color, a semiconductor material 13 containingspecific impurities used to form the photodiode 14, a first transparentbuffer layer 18 to be inserted between the color filter 12 and thesemiconductor material 13 so as to facilitate the formation ofstructures and improve the light transmittance, and the photodiodecontaining impurities having an impurity type opposite to that of thesemiconductor material 13.

In addition, a conductor pad 17 for external connection is formed in thefirst wafer portion 10 according to the present invention.

The second wafer portion 20 will now be described in detail.

The second wafer portion 20 is divided into the pixel array regionhaving a 3T or 4T structure and the peripheral circuit region. Theperipheral circuit region has a typical image sensor structure.Accordingly, the peripheral circuit region may include a circuit forreading an image sensor signal, a CDS circuit, a circuit for processinga common analog signal, a digital control circuit, and an image signalprocessing digital circuit.

In the pixel array region, pixel elements except the photodiode whichconstitute the pixel are regularly arranged. An example of this is shownin the lower part of FIG. 12. First, the pixel array region includes aconductor pad 21 for receiving a signal from the upper part of the pixelarray region, a reset transistor for initializing the photodiode 14, asource follower transistor 24 for transmitting voltage statuses of avoltage source VDD and a floating diffusion region 15, that is afloating node, to outside, a selection transistor 25 for controllingconnection between a pixel and an external lead-out circuit to transmitinformation of the pixel, and an output electrode 26 of the pixel.

FIG. 15 shows a separation type unit pixel for a 3-dimensional imagesensor according to another embodiment of the present invention.

The unit pixel of FIG. 15 further includes a microlens 11 condensinglight onto the photodiode and a second transparent buffer layer 19 whichis inserted to facilitate the formation of structures and improve thelight transmittance. The second transparent buffer layer 19 isadditionally used and corresponds to the membrane of a common imagesensor.

FIG. 16 is a flowchart of a manufacturing method of a separation typeunit pixel having a 3-dimensional structure for an image sensoraccording to the present invention.

First, a first wafer is constructed with only a photodiode formed byimplanting impurity ions into the semiconductor substrate (operationS611).

In the process of constructing the first wafer portion, a second waferportion is constructed to have the pixel array region including atransmission transistor, a reset transistor, a source followertransistor, and a blocking switch transistor and the peripheral circuitregion including a lead-out circuit, a vertical/horizontal decoder, aCDS circuit which involves in a sensor operation and an image quality,and analog circuit, an analog-digital converter (ADC), and a digitalcircuit (operation S612).

Second, the first wafer and the second wafer are arranged up and down(operation S620).

To arrange the first wafer and the second wafer up and down, the waferportions may be arranged in an optical manner by making a hole in thefirst wafer by using an infrared ray (IR) penetrating method, an etchingmethod, or a laser punching method.

In the IR penetrating method, the wafer are disposed without making ahole in the first wafer. In the etching method or the laser punchingmethod, a hole is formed through the first wafer, and then the wafer arearranged through optical pattern recognition.

Third, the first and second wafer arranged up and down are adhered to aconductor pad (operation S630).

Fourth, the surface thickness of the back side of the first waferportion is reduced to form thin back side of the first wafer (operationS640).

After the first wafer is adhered to the second wafer, the back side ofthe first wafer is thinned to reduce the wafer thickness. In order toreduce the thickness of the back side of the first wafer, the back sideof the wafer is processed by a grinding process, a chemical mechanicalpolishing (CMP) process, or an etching process.

Fifth, a color filter is formed on the first wafer (operation S650).

Sixth, a microlens is formed on the color filter (operation S660).

FIG. 17 shows an arrangement of the first and second wafer whenmanufacturing a separation type unit pixel having a 3D structure for animage sensor, according to an embodiment of the present invention.

The first and second wafer 10 and 20 are accurately aligned by using theIR penetrating method, the etching method, or the laser punching method.

In FIG. 17, a hole is formed through the first wafer 10 by using theetching method or the laser punching method.

The hole is not formed through the first wafer in using the IRpenetrating method.

The manufacturing method of a separation type unit pixel having a 3Dstructure for an image sensor of the present invention is not limited toa CMOS manufacturing process, and the method may be used in othersemiconductor manufacturing processes.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

INDUSTRIAL APPLICABILITY

Accordingly, the present invention has advantages in that, manufacturingprocesses can be simplified by constructing the upper wafer using only aphotodiode and the lower wafer using the pixel array region except thephotodiode, and costs are reduced since transistors are not included inthe upper wafer, which in turn cannot affect the interaction with light.

In addition, the present invention has advantages in that, by forming anarea for a photodiode almost the same as an area for a pixel, an imagesensor can be manufactured to have a good sensitivity in a subminiaturepixel, without a microlens. In addition, by disposing the photodiode atthe top layer, an incident angle margin of incident light can besecured, which has to be basically provided by the sensor for its autofocusing function or zoom function.

1. A separation type unit pixel having a 3D structure for an imagesensor comprising: a first wafer including a photodiode containingimpurities having an impurity type opposite to that of a semiconductorsubstrate and a pad for transmitting photoelectric charge of thephotodiode to outside; a second wafer including a pixel array region inwhich transistors except the photodiode are arranged regularly, aperipheral circuit region having an image sensor structure except thepixel array, and a pad for connecting pixels with one another; and aconnecting pad connecting the pad of the first wafer and the pad of thesecond wafer, wherein the first wafer comprises: a semiconductorsubstrate which contains specific impurities for forming a photodiode; afirst transparent buffer layer which is inserted between a color filterand the semiconductor substrate to facilitate formation of structuresand to improve light transmittance; a photodiode which containsimpurities having an impurity type opposite to that of the semiconductorsubstrate; and a pad transmitting photoelectric charge of the photodiodeto outside.
 2. The separation type unit pixel having a 3D structure foran image sensor of claim 1, wherein the first wafer comprises: a colorfilter which allows each pixel to display a specific color; a microlenswhich collects light to be condensed onto the photodiode; and a secondtransparent buffer layer which is inserted between the microlens and thecolor filer to facilitate the formation of structures and to improvelight transmittance.
 3. The separation type unit pixel having a 3Dstructure for an image sensor of claim 1, wherein, in the pixel arrayregion of the second wafer, the rest of the circuit elementsconstituting a pixel such as a transmission transistor, a resettransistor, a source follower transistor, and/or a blocking switchtransistor are arranged regularly.
 4. The separation type unit pixelhaving a 3D structure for an image sensor of claim 1, wherein theperipheral circuit region comprises a circuit for extracting an imagesensor signal, a CDS circuit, a circuit for processing a common analogsignal, a digital control circuit, and an image signal processingdigital circuit.